The old 7.3 diesel motors are great when they run, their biggest fault is in the glow plug control relay (passenger side fender well), wiring for the glow plugs (harness) and the glow plugs themselves. If you are sure of wiring harness and the plugs, the relay would be the place to start. Hope this helps, and if you have any more questions please let me know.Thanks,Joey

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When you turn the key on you should beable to hear the fuel pump come on. Other than that you have a fuel pressure test port, looks just like a tire valve stem get a little probe press the valve after cranking fuel should come out strongly and not dribble out. If is doesnt seem to be firing your going to have check the coil pack for spark from the coil pack take one plug wire out and put it in a position that as you crank the vehicle you will be able to see spark jump or even hear give it a 1/4 air gap from its connection. If you dont have spark and your timing chain isnt broken you need to check for power the choice of tool is a test light.or volt meter. If you dont have either test tool we will go by sound when you turn the key on the engine fuse box has relays do you hear them click. That click noise will be fuel pump relay and PCM relay for power inpect those relays for corrode connections you mainly have a power problem and inspect for proper ground a missing ground can also cause the ECM to not fire the coils

I don't believe any VW sent to the US in the past 15+ years has come with a plug-in heater. The only exception is if the car was from Canada, or if one was installed afterwards.

If the engine turns too slowly to start, try cleaning the terminals of the battery, if still nothing- replace the battery.

If the engine turns over but just won't fire up cycle the glow plugs a few times before trying to start. Turn the key to on, wait for the glow plug light to go out after a few seconds then turn the key off. Repeat 2 or 3 times before you try to start.

Also make sure you use a fuel additive for the cold months. Look for something with anti-gel on the label.

if you need connectors and terminals for the glow plug connections under the valve cover, Ford sells then so you don't need a whole $600 harness.

info for your use

Section 03-07B: Glow Plug System
1996 F-250, F-350, F-Super Duty Equipped with 7.3L Diesel Engines Workshop Manual
DESCRIPTION AND OPERATION
Glow Plug System
The 7.3L diesel engine uses a glow plug system which preheats air in the combustion chamber to improve cold engine starting.
The system consists of eight glow plugs, the glow plug control module (12B533), powertrain control module (PCM) (12A650), engine oil temperature (EOT) sensor and barometric pressure sensor (BARO sensor) (12A644).
The glow plug system is electronically controlled by the powertrain control module. The powertrain control module energizes the glow plugs immediately after the key is placed in the ON position and then determines how long the glow plugs will be on according to the engine oil temperature and the barometric pressure. The required time for the glow plugs to be energized decreases as the engine oil temperature and barometric pressure increase.
The glow plugs are self-regulating, and the powertrain control module controls relay ON time.
The powertrain control module protects the glow plugs by energizing them for short durations if the battery voltage is abnormally high.

Possible Causes:
Insufficient glow plug ON time will not allow enough heat to accumulate in the combustion chamber to easily facilitate starting. If the glow plug system ON time does not meet any of the specifications in the accompanying chart the problem is most likely a faulty wire harness connection, ground connections or glow plug relay.

If the glow plug resistance to ground is high, the most likely causes are an open UVC (under valve cover) harness or open glow plug.
--------------------------------------------------------Relay Operation
Install a DVOM on the glow plug feed side of the glow plug relay (large stud with two wires connected). Turn the ignition key to the ON position but do not attempt to start. Note the time in seconds from when the key is turned on and the glow plug relay energizes until the glow plug relay de-energizes. The glow plug relay makes a loud click noise which is easily heard when it energizes and de-energizes. The dome light will dim and the dash voltmeter will dip when the glow plugs are drawing current from the battery. Compare the times measured to the table (time will be affected by engine temperature, battery condition and vehicle altitude). The voltage at the glow plug feed terminal may vary from 9 to 12 volts depending upon battery condition.
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If battery voltage not present check for B+ at power supply terminal (terminal with single large wire). Power for glow plug power supply is supplied from starter relay through two fusible links at solenoid. Glow Plug Operation
Disconnect all of the glow plug/injector harness connectors from the valve cover gaskets. With the pigtail installed, measure glow plug resistance to ground (preferably B-). A resistance measurement of .1 to 6 ohms indicates a good glow plug.

Glow Plug Harness Continuity
Measure for continuity from the connector harness to the glow plug feed terminal on the glow plug relay. Resistance should be less than 5 ohms. Incorrect measurements will result if all glow plug/injector connectors to valve cover are not disconnected.

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Glow Plug Relay
The glow plug relay (12A343) has a plastic base and two mounting bolts. It is operated by the powertrain control module (PCM) (12A650) which senses engine oil temperature, PCM voltage and barometric pressure.
When the engine oil temperature or the barometric pressure is low, the PCM activates the glow plug relay and the glow plugs.
If the PCM voltage is too high, the powertrain control module deactivates the glow plug relay and the glow plugs immediately energize for a shorter duration, decreasing the modulation of the duty cycle. -----------------------------------------------
Glow Plug
Removal

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Glow Plug Control
Glow plugs are used to warm the cylinders to improve cold-engine starting. The PCM uses the engine oil temperature (EOT) sensor and barometric pressure (BARO) sensor to determine how long the glow plugs will be on. The PCM energizes the glow plugs longer if the engine is very cold or if the barometric pressure is low at high altitudes.
The PCM controls the relay that supplies power to the glow plugs with the glow plug control (GPC) signal. When the GPC signal is grounded by the PCM the relay closes, turning the glow plugs on.
The glow plugs are controlled by the following sequence:

PCM energizes the glow plugs.

The PCM energizes the glow plugs immediately after the ignition is placed in the ON position.

The PCM then determines how long glow plugs will be on based on EOT sensor and the BARO sensor. The required time to warm up the cylinder decreases the engine oil temperature and barometric pressure increases at low altitudes.

Glow Plug Lamp Control
The glow plug lamp (GPL) signal controls the WAIT TO START light located on the instrument panel. This light is used to indicate when to start the engine. The cylinders are being warmed by the glow plugs during and after this light is lit. The PCM energizes the glow plug lamp longer if the engine is very cold or if the barometric pressure is low.
When the GPL signal is brought low, the "Wait to Start" lamp is turned on.
The glow plug lamp is controlled by the following sequence:

PCM lights the WAIT TO START light after a key on reset occurs.

The PCM determines the glow plug lamp activation time based on engine oil temperature, barometric pressure and battery voltage.

PCM turns off the WAIT TO START light.

The glow plug lamp is turned off when the timer counts to the number of seconds specified by the PCM. The glow plugs will normally remain on longer than the "Wait to Start" lamp.
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